Catalogue No. - Date - Times Played
Supose we have a table whihc name is Play history and its have a three colums Catalogue No and Date anf Times Play.These are three Colums in a 1 row.
In this situation the Catalogue No. number is a Primary Key in the CDs table but only part of the Composite Key in the Play History table as the date is required to form the whole key. An alternative could have been to create a new unique single-attribute Primary Key for the table. As we still need to know the catalogue number in order to work out how many times a DJ has played Britney, Christina or Pink. The catalogue number would become the Foreign Key in the Play History table,that is Foreign Key is a attribute or column in a table that refers to a Primary Key in another table.